In brief, shot peening is a cold metal-working process that produces a compressive residual stress layer in a subject metal component and concurrently modifies the mechanical properties of that metal component. Shot peening comprises impacting a surface of the subject metal component with round metallic, glass, or ceramic particles, commonly referred to as “shot.” The force at which the shot strikes the surface of the subject metal component, and correspondingly produces a compressive residual stress layer in the subject metal component, is application-specific.
It is well known in the art that the process of shot peening increases fatigue strength in the subject metal component by relieving tensile stresses that contribute to stress-corrosion cracking. In conventional shot peening processes, spheroid particles of cast steel, cast iron, glass, etc., are blown or mechanically impelled in a high velocity stream against the surface to be treated. The individual shot particles produce shallow, rounded overlapping dimples in the surface, stretching it radially from each point of impact and causing cold working and plastic flow. The resultant compressive stress tends to counteract tensile stresses imparted to the surface by preceding rolling, bending, abrading, and similar processes that occur during the manufacturing of the subject metal component.
Two critical shot peening factors are “peening intensity” and “coverage.” Peening intensity is a function of the weight, size, hardness and velocity of the peening particles, angle of impingement, and various other factors. Accordingly, peening intensity is a function of the kinetic energy of the shot impacted upon the surface of the component. In summary, it is a function of shot velocity and size. Shot is accelerated by using air pressure to force it through a peening nozzle or accelerated centrifugally via a wheel and directed at the component's surface. Coverage rate or the rate of approach to saturation depends on dimple size, the rate of dimple accumulation and statistics.
The use of a test strip or coupon, such as for example an Almen strip, is well known for measuring shot peening intensity and has been referred to as a key element in shot peening process control. Developed and patented by John O. Almen, Almen strips comprise thin metal strips that deflect when subjected to shot peening. A test strip or coupon, or an Almen strip, is mounted in a special holder wherein a critical section of the strip is exposed. Thus, the Almen strip mounted in the special holder is exposed to the shot stream under substantially the same conditions as the subject metal component undergoing peening.
Almen strips deflect in response to the surface compression produced by shot impacts. One impact causes some deflection of the strip toward the side struck. As the impacts accumulate, the deflection increases. Deflection is also greater if the impacts are more energetic, for example comprising higher shot velocity and diameter. The Almen strip is a convenient way of assessing the overall peening process.
An Almen strip is clamped in a special fixture and peened under a given set of conditions, such as, gun air pressure, shot size and shot mass flow rate. The deflections of many strips peened under these conditions and for varying amounts of time are then plotted on what is known as a saturation curve. Saturation is said to occur when doubling the peening time has a 10% effect on deflection. For practical purposes, at saturation the strip is fully covered with impact dimples so its deflection is proportional only to the peening intensity. Generally, peening of the subject component parts may begin at this saturation intensity.
Almen strips are classified into three (3) types: “A”, “N” and “C”. They differ in thickness and define the same width and length. Typical Almen strips are made from SAE 1070 cold rolled spring steel and have surface dimensions of 0.75 inch×3.00 inches. Almen strips are commercially available in three thicknesses: 0.031 inch (“N”-strips); 0.051 inch (“A”-strips); and 0.093 inch (“C”-strips).